1. Field of Invention
The present invention relates to a magnetic restoring device of an actuator, and more particularly to an objective lens driving device in an optical pickup.
2. Related Art
Conventionally, in an optical disk device using an optical disk as a recording medium, an optical pickup is used for reading out information pits in which information signals are written on disk. In such optical pickup it is necessary to focus precisely a laser beam to these information pits on the disk. For this purpose, an objective lens driving device is used, which moves an objective lens two-dimensionally in the direction of an optical axis, i.e., according to a focusing signal, and in the tracking direction against deviations or oscillation of the disk. At least four properties are required to this objective lens driving device, such as 1) having a high sensitivity, 2) being excellent in dynamic characteristics such as the ability of following in a high frequency region, 3) being small, and 4) being not expensive.
A shaft/sleeve based actuator for positioning is widely employed in a conventional driving device of optical pickup. The position of a movable part of actuator has the tendency of moving down due to the effect of gravity. As such, a number of techniques have been proposed for maintaining a movable part of actuator in a neutral position, i.e., an equilibrium position when no force is applied on the movable part. For example, U.S. Pat. No. 4,792,935 discloses adding a resilient support member on the movable part of actuator. But this is unsatisfactory for increasing the difficulty of assembly due to the adding of additional support components. Another related art is disclosed in U.S. Pat. No. 4,998,802 which causes the movable part of actuator returning to neutral position by designing that a magnetic attractive force of the magnetic portion for focusing acts on a thin magnetic piece and at the same time a restoring force acts thereon, which is about equal to the restoring force so that the magnetic piece has a tendency to be stably held at the maximum point of the magnetic flux. But this is unsatisfactory due to attractive force and restoring force are not about equal each other if the magnets have a poor quality in forming a thrusting force on the support journal, thereby increasing friction force when magnetic piece is actuated. Further, the magnetic restoring force is not sufficient. Another related art proposes an improved technique for keeping magnetic piece in an equilibrium position. However, no proposition has been provided to increase magnetic restoring. As far as we know, a sufficient magnetic restoring force is a must for smoothly operating the actuator. Thus a need exists for solving the above drawbacks of related art.
It is an object of the present invention to provide a magnetic restoring device of an actuator having the capability of providing a sufficient magnetic restoring force for stably holding a movable part of a actuator in a neutral position.
The advantages of the present invention are realized by providing a novel actuator acting as an objective lens driving device in an optical pickup comprising a lens holder, a voice coil motor, two magnetic inductive members, and two yokes. The lens holder has a support journal for supporting the objective lens. The voice coil motor includes a base, a focusing coil provided on lens holder, a tracking coil provided on the lens holder, and a plurality of magnets provided on the base such that an electromagnetic effect is formed when a driving current is made to flow through the focusing and tracking coils respectively which being exposed to the influence of a magnetic field of a corresponding magnetized magnet, thereby moving the lens holder along the support journal to effect a focusing operation for the optical pickup, and oscillating the lens holder along the support journal to effect a tracking operation for the optical pickup. The magnetic inductive members are provided between the magnets and the yokes to form a magnetic circuit by a magnetized magnet and an adjacent corresponding yoke such that a magnetic restoring force is generated by the magnets for acting on the magnetic inductive member located therebetween. The lens holder accordingly is held at a neutral position by the magnetic restoring support of the magnetic inductive member. The movable part of the actuator is able to cause lens holder to return to its neutral position after the focusing and tracking operations. Further, magnets and yokes are opposite. One arc-shaped surface is formed on the magnet in the focusing direction, and the other one is formed on the magnet in the tracking direction. As such, an increased magnetic restoring force is obtained in the focusing and tracking directions respectively by the magnetic restoring device. By utilizing this, a couple of drawbacks occurred in related art are eliminated such as a thrusting force on the support journal causing an increased friction force when magnetic piece is actuated, and not sufficient magnetic restoring force.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.